It has recently been discovered that glia become progressively more activated upon repeated exposure to morphine, & that this glial activation, in turn, modulates morphine's effects. This discovery was originally made in the context of studying the pain suppressive effects of morphine in spinal cord. The possibility that glia may be fundamentally important in determining the effects of opioids such as morphine is novel & important in its implications. Because of this, we propose to explore whether glia may profoundly alter the effects of repeated morphine in brain, as well. We believe that glia will prove to be powerfully involved in several phenomena currently thought to arise purely as a result of opioid effects on neurons; that is, dependence/withdrawal, reward & aversion. If this were true, it would provide evidence that glia are critically involved, not only in modulating the pain-suppressive effects of opioids, but also in key phenomena associated with human opioid abuse & addiction. Having considered the clinical importance of opioid dependence/withdrawal, reward & aversion & the paucity of knowledge regarding glial involvement in any of these, we have chosen to first determine the range of opioid-related phenomena in which glia & glial products play an important role, before detailed analyses of any single phenomenon. We chose this strategy as it is the right approach for this stage of investigation. The data to be obtained will provide guidance as to which of these phenomena should later be studied in detail, including issues of generality to other opioid & non-opioid drugs beyond morphine. The aims of the proposal are: (1) To define whether glial activation contributes to the development &/or maintenance of morphine dependence. This will be accomplished by characterizing whether inhibition of glial activation before or after establishment of opioid dependence impacts the expression of morphine withdrawal. Whether glia contribute to CNS side effects associated with dependence-inducing opioid administration will be assessed as well. (2) To characterize changes induced in glia by in vivo morphine dependence/withdrawal. Here, (a) brain regions implicated in the effects of repeated morphine & (b) rapidly isolated microglia will each be examined using immunohistochemical, protein &/or mRNA analyses. (3) To define whether glial activation contributes to morphine reward or aversion. This will be accomplished by characterizing whether inhibition of glial activation disrupts conditioned place preference &/or conditioned place aversion. In these studies, inhibition of glial activation will be accomplished in 2 ways: (a) using 2 blood-brain barrier permeable glial activation inhibitors & (b) blocking the recently discovered non-classical opioid receptor on glia.